The operators of the tsunami-stricken Fukushima nuclear power plant and government regulators were woefully unprepared for disaster, the first official probe into the March 11 catastrophe said Monday.

An independent panel set up to investigate the events around the world's worst nuclear accident in a generation said Tokyo Electric Power (TEPCO) and the Nuclear and Industrial Safety Agency had failed to think beyond the risks for which the nuclear plant was designed.

"TEPCO did not take precautionary measures in anticipation that a severe accident could be caused by (a) tsunami such as the one (that) hit... Neither did the regulatory authorities," the report said.

"TEPCO did not incorporate measures against tsunamis exceeding the design basis. This indicates the limit of voluntary safety measures."

The panel's interim report sharply contrasted with TEPCO's own probe, which said the utility could have had no way to prepare for the 9.0-magnitude earthquake and giant waves that sparked meltdowns and explosions at the plant.

The 500-page document, based on interviews with 456 people involved in the accident, comes nine months after the crisis forced the evacuation of tens of thousands of people as radiation leaked into the air, sea and food chain.

The panel said its aim was not to apportion blame for the disaster, but to learn why the accident happened in the way it did.

TEPCO's accident management programmes assumed only relatively minor internal incidents such as mechanical failures and human errors, the panel said.

Risks such as "earthquakes and tsunamis were not included in the scope of consideration," the panel said, adding that regulatory bodies also failed to force TEPCO to establish thorough safety measures.

"Measures against severe accidents should not be left with the operators voluntary activities," the provisional English translation of the report said.

"The nuclear safety regulatory bodies should consider and determine legal requirements when they deem necessary."

Emergency procedures set out by TEPCO and the government were not followed and were found to be impractical, the report said.

For example, Fukushima workers mistakenly believed that the plant's cooling system was working even after the tsunami actually knocked it out. They then missed opportunities to correct that misunderstanding, the report said.

The error caused delays in the pouring of water into overheating reactor No. 1, which eventually went into meltdown and exploded, it said.

Communication failures among crews working on site left small groups of individuals to make their own decisions and meant that actions were not reported to supervisors.

Such errors also caused delays in the cooling of reactor No. 3, the panel said.

"Collection of accurate and most up-to-date information is a prerequisite for timely and appropriate decision-making. This issue, together with the need for providing information to the nation, is of a major concern," the report said.

TEPCO's "preparedness for such (an) accident as severe (as) damage at the core of (a) reactor as a result of natural disasters was quite insufficient".

The public's mistrust in the government grew as Tokyo repeatedly stressed that despite nuclear explosions, health risks were low, the panel said.

The government decided to withhold simulations of how radioactive materials from the wrecked plant could spread, while repeatedly playing down health worries.

"If the information were provided (in a timely way), it could have helped local governments and populations to choose (a) more appropriate route and direction of evacuation," the panel said.

The government also failed to use the simulation for itself due to miscommunication among officials, it said.

TEPCO and government officials did not ensure information flowed during the crisis, with bureaucrats failing to ask about the reactors, relying on the utility to report to them instead.

The panel also pointed to insufficient communication among government officials themselves.

It said a crisis management headquarters, headed by the premier, was launched in a fifth-floor office of the prime minister's residence, while working-level officials gathered at the crisis management centre in the basement of the same building.

The final version of the report is due to be published in summer 2012.